1. Field of the Invention
The present invention relates to a multi-layer paper sheet forming system applicable to a paper making machine.
2. Description of the Prior Art
One example of a multi-layer paper sheet forming system in the prior art is illustrated in FIG. 9, which is a front view of a wet end of a short wire cloth duplex paper making machine. In FIG. 9, stock liquor ejected from a head-box 015 is formed into a paper layer by means of a conventional table instrument 016 (not shown in detail) disposed on a short wire cloth 017. This paper layer is engaged with an endless felt 018 on a forming cylinder 20, and while it is being dewatered by an urging pressure of the felt, it is combined with a paper layer formed in a preceding unit and having been carried by the felt. Under the state of being sandwiched between the felt and the short wire cloth, it is carried to a section of a turning roll 021, where bonding of both the above-described paper layers is effected by pressing with a turning touch roll 024. Thereafter, the wet paper layer is conveyed to the next unit loaded on the felt.
FIG. 10 shows another example of a multi-layer paper forming system in the prior art. The system of this example attempted to enlarge a dewatering capability by constructing a paper layer forming apparatus making use of two wire cloths in a short wire cloth section. Stock liquor ejected from a headbox 015' is dewatered by a conventional table instrument 016' (not shown in detail) disposed within a loop of the short wire cloth 025 and thereby a paper layer on the under side is formed.
Next, the paper layer is dewatered to the outside by wire tension of another short wire cloth 026 coming into engagement on a forming cylinder 020' and centrifugal force, and thereby an upper side portion of the paper layer is formed. Thereafter, the wet paper layer riding on the short wire cloth 025 is bonded with a paper layer formed in a short wire cloth section of the preceding stage and carried by a felt 018' by means of a couch roll 027 at the location where the same short wire cloth 025 is engaged with the felt 018'. The thus combined multi-layer paper sheet is either picked up by a suction pick-up roll 028 or transferred to the next stage (press part) by open-drawing.
FIG. 11 shows still another example of a multi-layer paper sheet forming system in the prior art. The system shown in FIG. 11 is a wet end of a general composite paper machine making use of a circular wire cylinder 022a shown in FIG. 12 or a suction cylinder 022b (circular wire wound) shown in FIG. 13, and it is shown in a front view. As shown in FIGS. 12 and 13, a headbox 021' is covered by a curved roof 021'a by a certain length along the circumference of the cylinder, stock liquor fed into a gap formed between this curved roof 021'a and the cylinder 022a or 022b is dewatered on the dewatering cylinder, and thereby a paper layer is formed. Then it is engaged with a wet paper sheet carried by a felt 032' on the cylinder 022a or 022b and they are combined together by a couch roll 326.
The thus combined multi-layer paper sheet is picked up by a suction pick-up roll 041' and transferred to the next stage (a press part). It is to be noted that reference numeral 026' in FIGS. 12 and 13 designates a couch roll.
In the above-described multi-layer paper sheet forming system in the prior art shown in FIG. 9, since formation of a paper layer is effected by the instruments called foils and table rolls, the dispersion power of the raw material fibers is weak, and so formation is poor. In addition, upon making a high grammage (basic weight) paper sheet at a high speed, since an endless felt is pressed on a cylinder having a small radius of curvature, the pressure becomes excessively high, and hence there was a shortcoming in that there occurred the so called crushing of the wet paper sheet, caused by poor dewatering of stock liquor on the forming cylinder 020.
In the case of the multi-layer paper sheet forming system shown in FIG. 10, although there was merit in that, because of a high air permeability of the wire wrapped around the forming cylinder 020', the dewatering pressure is mitigated, also the dewatering capability is increased, and hence the system can be better adapted to a high speed and a high grammage than the system shown in FIG. 9, there was also a shortcoming in that because of a high wet paper concentration before combination, the bonding strength was low. In addition, because of the fact that the short wire cloth unit is formed of two wire cloths, the unit length became long, the initial cost was also high, and a large installation area was necessary.
In the case of the multi-layer paper sheet forming system in the prior art shown in FIG. 11, since the dewatering was by a fixed pressure consisting of the pressure applied to the stock liquor sandwiched between the cylinder 022a or 022b and the curved roof 021'a of the headbox 021', pressure caused by the pressing force of the couch roll, and further a vacuum force applied to the cylinder 022a or 022b, there was a shortcoming in that a degree of dispersion of fibers under the state of forming a paper layer was low, and so the formation was poor. Furthermore, if an amount of deposition per unit is enlarged or the operation becomes high-speed, then a concentration of the wet paper coming into the combining section is lowered, and hence there occurs the problem of crushing of the wet paper by a nip pressure of the couch roll on the circular wire cloth cylinder 022a or the suction cylinder 022b. So it was impossible to choose a large amount of deposition per unit.
FIG. 14 shows yet another multi-layer paper sheet forming system proposed in Japanese Patent Application No. 5-20823 (1993) in order to overcome the shortcomings of the heretofore known multi-layer paper sheet forming system shown in FIGS. 9, 10 and 11. In this system, a plurality of short wire cloth sections 055, in which a dewatering instrument 054 is equipped within a loop of a short wire cloth 055', are disposed along a lower traveling path of an endless felt 056, and the endless felt 056 is made to travel on the dewatering instrument 054 jointly with the short wire cloth 055' so that in each short wire cloth section 055 stock liquor may be ejected from a headbox 053 into the space between the endless felt 056 and the short wire cloth 055'.
However, in this system, since a part of the headbox 053 projects above a breast roll 057, it is necessary to make the endless felt 056 travel so as to avoid it, and accordingly there was a problem that since the arrangement of the plurality of short wire cloth sections 055 formed a step-like shape, if the number of the short wire cloth sections became large, the base would become complicated and expensive. It is to be noted that in FIG. 14, reference numeral 054a designates a shoe blade, numeral 058 designates a turning roll, numeral 059 designates a stretch roll, numeral 060 designates a guide roll, numeral 061 designates a lead-in roll, numeral 062 designates a couch roll, numeral 063 designates a suction couch roll, numeral 064 designates a felt roll, numeral 065 designates a flow-back device, and numeral 066 designates a suction pick-up roll.